Carl Zimmer has written about the prevalence of metamorphism in the animal world much more common than I had ever imagined.
He writes about the work of Hanna ten Brink in the Netherlands, which begins from the conservative framework of most evolutionary biologists today, the selfish gene ideology. She starts from the premise that animals maximize their individual fitness, meaning they try to reproduce as much as possible. Her conclusion, then, is that animal species can eat more total food if they have access to two different ecological niches.
...animals pay a steep price to go through metamorphosis. They burn a lot of calories to tear apart the old anatomy and develop a new one. There's a chance that this complicated process will go awry, leaving them with defects.
Metamorphosis also takes time, leaving animals vulnerable to predators and parasites. In many cases, Dr. ten Brink and her colleagues found, the cost of metamorphosis is too high for it to be favored by natural selection.
"You have to get back something really good," she said.
In my reearch, I have been skeptical of the selfish gene ideology, and I have promoted the radical thinking of Lynn Margulis. I think Lynn is correct that the way metamorphosis evolves is not by one species reaching out into two niches, but by two entirely different species merging their genomesperhaps hard to imagine on its face, but Lynn cites a "paper trail" from the genome.
And what is the fitness payoff for the merger? My theory is that it solves an ecological problem. Adults are much larger, stronger, more experienced and more robust than their offspring. It is hard for the young to grow up if to do so they must compete with larger and stronger versions of themselves. Metamorphosis is a way to take the young out of competition with their elders, who have an unfair advantage. It is thus about preserving the species, not the individual.